Cantilever stator blades in which shrouds are provided as separate pieces, shrouded stator blades in which shrouds are integrally provided, and so on, are typically employed as stator blades of gas turbine compressors.
With the shrouded stator blades, leakage of air, etc. is less likely to occur at tips of airfoil portions thereof as compared with the cantilever stator blades and, in addition, a rotor seal structure that suppresses leakage of air, etc. between the stator blades and the rotor can be provided at the inner circumferences of the shrouds. This allows the shrouded stator blades to reduce the air leakage level to an appropriate amount; therefore, they are considered advantageous in terms of performance.
In the above-described shrouded stator blades, circumferential base portions referred to as shroud portions are provided at outer and inner end portions of the airfoil portions (profile portions).
Examples of methods for securing the airfoil portions to the shroud portions include the tenon-type securing method, in which insertion portions protruding from the airfoil portions are inserted into insertion openings provided in the shroud portions, and the pork-chop-type securing method, in which insertion-flange portions formed in a widening shape from the airfoil portions are inserted into the above-described insertion openings.
With the tenon-type securing method or the pork-chop-type securing method, the insertion portions or the insertion-flange portions may be secured by mechanically inserting them into the insertion openings, or they may be secured by brazing or welding. The shroud portions of the stator blades are assembled into a ring shape in this way.
In addition, in some cases, the airfoil portions and the shroud portions are molded or machined as an integral structure.
In order to absorb thermal expansion in the circumferential directions in a ring-shaped assembled state, to enhance the ease of machining and assembly of the shroud portions, and to achieve enhanced ease of maintenance for the shroud portions, etc., the shroud portions are typically divided into a plurality of portions in the circumferential direction. For example, in the case of the shrouded stator blades, the shroud portions are divided in correspondence with each stator blade.
Furthermore, a seal structure, such as a labyrinth seal, a honeycomb seal or the like, is provided between the shroud portions and a rotating rotor shaft (for example, see Patent Literature 1).
In consideration of the ease of machining or the ease of repairing, the configuration of the seal structure may be such that the seal structure is formed as a separate structure from the airfoil portions or the shroud portions, wherein the seal structure is combined with the airfoil portions or the shroud portions after being formed.
In addition to the structure disclosed in Patent Literature 1, examples of configurations in which the shroud portions are combined with the seal structure include a configuration in which a seal structure is fitted to groove structures provided in shroud portions.
On the other hand, in a flow field of air or gas inside a compressor of a gas turbine, it is known that when stator blades receive an excitation force having a frequency matching the natural frequency of the stator blades or a frequency that is an integral multiple of the rotation speed, the airfoil portions and the shroud portions of the stator blades exhibit large vibrations (exhibit a vibration response).
Examples of the above-described excitation force include the excitation force of a wake flow (wake) of rotating rotor blades, the excitation force of an interference flow (potential), and so forth.
When the stress that acts on the stator blades caused by the above-described vibration response increases, exceeding the fatigue resistance of materials that constitute the stator blades, fatigue cracks may form in the stator blades, and the stator blades may be broken due to the fatigue cracks.
Because of this, it is necessary to design the airfoil portions and the shroud portions so as to have physical-frame strength that prevents fatigue crack formation even if the vibration response occurs, and the natural frequency of the stator blades also needs to be shifted, in other words, detuned, from the excitation frequency that is expected to act on the stator blades.
On the other hand, along with increases in output power, enhancement of performance, and reduction of costs in gas turbines in recent years, the size of profile portions is being increased, including enlargement of the blade profile width (blade chord), enlargement of the blade length (span), and so forth in the profile portions.
When the profile portions are increased in size in this way, the aerodynamic force or force of gas that acts on the airfoil portions increases, and the load or moment that acts on base portions of the airfoil portions, in other words, connection portions between the airfoil portions and the shroud portions, increases. In order to endure such increases in load or moment, sufficient strength needs to be ensured by increasing the radial size of the radius of curvature R of fillets formed at the base portions of the airfoil portions.
With regard to this, in contrast to ensuring sufficient strength at the base portions of the airfoil portions, there is a demand from an aerodynamic standpoint, that it is preferred to reduce the radial size of the radius of curvature R of the fillets formed at the base portions of the airfoil portions.
The profile portions compress gas-containing air, etc. by being rotationally driven, and, on the other hand, receive air (containing gas) resistance in the flow field. Therefore, in order to decrease this air resistance, the profile shape is optimized, the leading-edge diameter and trailing-edge diameter in the profile portions are decreased in the radial sizes thereof, and the airfoil thickness itself is reduced.
However, the above-described reduction of the radial size or thickness is a factor that decreases the strength of the stator blades, in particular the strength against a resonant response. Accordingly, with regard to designs of the profile portions, there are restrictions on the above-described reduction of the radial size or thickness in order to ensure the strength of the profile portions.
In addition, in order to prevent the stator blades from breaking through resonating with the excitation force, the natural frequency of a stator-blade ring as a whole, in which a plurality of stator blades are combined, is shifted from the frequency of the excitation source; that is, detuning design is conducted so that the frequencies do not match.
However, because the above-described natural frequency depends on the shape of the profile portions, the shape of the shroud portions, and so forth, when detuning between the natural frequency and the frequency of the excitation source is given priority, the stator blades in many cases are inevitably designed at the expense of the aerodynamic characteristics of the stator blades.
Patent Literature 1 proposes a technique of pressing the stator blades with wave-shaped plate springs in order to restrict the relative movement of the stator blades.
Furthermore, in order to reduce the vibration response in the stator blades, there is also a known technique for damping vibrations due to the vibration response in the stator blades by vibration damping (damping) which uses a frictional force using springs.
More specifically, a known structure damps vibrations in the stator blades with a structure in which doughnut-ring shaped springs are inserted between a shroud ring that is disposed on an inner circumferential side and a seal holder that holds a seal, pressing the springs against the shroud rings.
By doing so, when the shroud portions articulated with the profile portions vibrationally deform due to resonance, the shroud portions and the springs slide, and a frictional force acts between the shroud portions and the springs. Consequently, vibrational energy is converted into frictional energy (thermal energy) at the sliding surfaces between the shroud portions and the springs, thus damping the vibrations of the stator blades.